Device for clearing a supply pipe of a reducing agent that has previously gone from a first phase to a second phase then back again

ABSTRACT

In a device for keeping a valve controlled flow passage for supplying a reduction agent to a catalytic converter in an exhaust system of a diesel engine, wherein the reduction agent is gaseous above a certain temperature but solid below this temperature, the valve being a control valve for dosing the flow of the gaseous reduction agent and including a valve seat and a movable valve control surface, the valve surface is formed by a controllable element which can be energized selectively to be cooled or heated so that, with the valve closed and the valve surface cooled to form a condenser element gaseous reduction agent in the flow passage is solidified at the valve surface but, with the valve surface heated is readily again decomposed so as to become gaseous. As the cooling of the valve surface forms adjacent the cooled valve surface a cold trap the resolidifed reduction agent precursor formed at the valve surface where it can easily be again decomposed by the heating of the valve surface.

This is a continuation-in-part application of international applicationPCT/IB2004/050469 filed Apr. 19, 2004 and claiming the priority ofGerman application 203 08 348.2 filed May 26, 2003.

BACKGROUND OF THE INVENTION

The invention resides in a device for cleaning a supply pipe of areducing agent which is used in the exhaust system of a diesel engineincluding an SCR catalytic converter.

In addition to carbon monoxide (CO), particles and hydrocarbons (HC) inparticular nitrogen oxides (NO_(x)) are the primary pollutants directlyemitted in the operation of internal combustion engines, particularlydiesel engines. Three-way catalytic converters, as they are used ingasoline and gas engines, can not be used with diesel engines because ofthe excess of oxygen in the diesel engine exhaust gas. For this reason,Selective Catalytic Reduction, SCR catalytic converters, which operateselectively, have been developed for the reduction of the nitrogen oxideemissions of diesel engines have been developed wherein the nitrogenoxide emissions often are reduced to N₂ and H₂O with the aid of addedreduction agents, namely ammonia (NH₃).

A device for supplying gaseous ammonia to the exhaust duct of aninternal combustion engine of a motor vehicle is known from DE 197 20209 C1. This device comprises a gas tight and pressure resistantconverter which includes a thermolytically NH₃ releasing compound or athermolytically NH₃ releasing material mixture, a so-called NH₃precursor. A NH₃ precursor, for example ammonium carbonate, may beprovided which is a solid material. The converter is connected to theexhaust duct of a diesel engine by way of a supply line which joins theexhaust duct in the flow direction of the exhaust gas ahead of the SCRcatalytic converter. As a dosing device, a clocked valve, is providedwhich is operated by a control unit in such a way that the NH₃ isintroduced into the exhaust gas flow in the required amount. Theconverter consists essentially of a pressure resistant reactioncontainer which is surrounded by a heating device in the form of heaterpipes. The heating device is connected to the cooling water circuit ofthe diesel engine via a supply and a return line.

Upon heating of the ammonium carbonate, which may for example be used asthe NH₃ precursor the ammonium carbonate breaks down into NH₃ and CO₂,whereby the reduction agent is converted from a first solid phase to asecond gaseous phase. The gas mixture is collected in the pressureresistant reactor container until a corresponding internal pressure hasbuilt up. When a certain internal pressure has been reached in thereaction container, an equilibrium state is reached so that noadditional ammonium carbonate is decomposed. Under the engine operatingconditions wherein the engine cooling water flowing through the heatingdevice normally has a temperature of between 800 and 100° C., thepressure in the reaction container in the equilibrium state is about 3-4bar. In order to provide a sufficient NH₃ amount for introduction intothe exhaust gas flow during dynamic operation of the diesel engine, theconverter also serves as a reaction gas storage device providing for acertain reserve volume for the reaction gas or, respectively the NH3reduction agent contained therein.

The reaction container is generally arranged at a certain distance fromthe exhaust duct. It has been found that at lower temperatures thereaction gas mixture may recombine in the supply pipe to the exhaustduct and return to the solid state depending on the type of precursorused. In the most disadvantageous case this may result in a blockage ofthe supply pipe. The same applies for the control valves arranged in thesupply pipe, such as the clocked valve used as a dosing device. However,the clogging of the supply line and particularly of the clocked valvedisposed therein driving shut down of the internal combustion engine isnot particularly problematic in connection with same reduction agentssuch as, for example, with ammonium carbonate. Since the NH₃ precursordeposited in the supply line, and particularly in the valve or valves,is again decomposed when the engine is again operated and temperaturesrise again so that the supply line and the valve or valves are againfreed. In order to render the supply line more rapidly operational, ithas already been proposed to heat the supply line and the valvesdisposed therein. However, depending on the distance of the reactioncontainer from the exhaust duct, the realization of such a heatingconcept concerning the whole supply line including the clocked valvesdisposed therein is quite expensive.

It is therefore the object of the present invention to provide anarrangement with which the problem of the NH₃ precursor deposition inthe supply line can be counteracted in a relatively simple way.

SUMMARY OF THE INVENTION

In a device for keeping a valve controlled flow passage for supplying areduction agent to a catalytic converter in an exhaust system of adiesel engine, wherein the reduction agent is gaseous above a certaintemperature but solid below this temperature, the valve being a controlvalve for dosing the flow of the gaseous reduction agent and including avalve seat and a movable valve control surface. The valve surface isformed by a controllable element which can be energized selectively tobe cooled or heated so that, with the valve closed and the valve surfacecooled to form a condenser element gaseous reduction agent in the flowpassage is solidified at the valve surface but, with the valve surfaceheated is readily again decomposed so as to become gaseous. As thecooling of the valve surface forms adjacent the cooled valve surface acold trap, the re-solidified reduction agent precursor formed at thevalve surface valve can easily be again decomposed by the heating of thevalve surface.

This device in the form of a dosing valve comprises a condenser elementforming a cold trap. The condenser element is connected to a controlunit and is operated as such with a corresponding control when theinternal combustion engine is shut down and/or when the valve is closed.The switching on of the condenser element may also depend on otheroperating parameters, for example, the momentary temperature of thevalve and/or of the supply line. Upon operation of the condenserelement, its temperature is reduced to form the cold trap with theresult that reduction compounds which are in the second phase (gaseous)are returned around the condenser element to the first phase (solid),for example with the use of ammonium carbonate or to an intermediatephase. In this process the reduction agents which are present in thesecond phase also in the area of the cold trap are pulled back into thecold trap and also converted into the first phase or an intermediatephase. By this effect reduction agents still present in the second phaseare removed from the supply line and any valves included therein or inclutches or similar compounds by the cold trap formed by the condenserelement as reduction agents which are contained at predeterminedlocations in the supply duct and are still in the second phase arecollected and regenerated. With a renewed use of the supply line forsupplying reduction agents to the SCR catalytic converter arranged inthe exhaust duct of the internal combustion engine then first orintermediate phase reduction agents deposited only in the area of thecondenser element need to be removed by an appropriate heating. For thisreason, the valve also includes a heating device so that also thereduction agent deposited in the valve by the activation of theconductor element can be reconverted to the second phase and supplied tothe SCR catalytic converter disposed in the exhaust duct of the internalcombustion engine.

The condenser element is arranged expediently either at a valve seat orat the movable valve which cooperates with the respective valve seat.The heating device is either assigned to the same valve element or therespective complementary valve element. The advantage of this device isthat it can at the same time act as a dosing valve. Preferred is anembodiment wherein the movable valve area of the valve is in the form ofa heatable condenser element so that this part of the valve can form thecold trap as well as the reconversion means of the reduction agents.Expediently, one or several Peltier elements are used for this purposewhich, dependent on how they are controlled, operate either to cool orto heat. It is advantageous in this connection if the movable valvecomponent itself represents the heat exchange surface of the Peltierelement or elements.

An embodiment wherein the stationary valve seat includes two adjacentconnecting openings which can be jointly closed by the movable valvestructure. The valve structure which is effective as a condenser elementand which closes both connecting openings when the valve is closedresults in the formation of a cold trap for each connecting line wherebyreduction agents present in the respective connecting lines in thesecond phases are particularly effectively withdrawn. One of theseconnecting lines is connected to a storage container including thereduction agent or a reduction agent precursor whereas the otherconnecting line of the valve is connected to the exhaust duct.

The arrangement described above is particularly suitable forinstallation in a supply line through which the gaseous reduction agentflows which has been generated from a solid NH₃ precursor by thermolyticdecomposition. Ammonium carbonate is particularly suitable as such aprecursor. The arrangement is also usable in connection with a systemwherein urea is used as NH₃ precursor.

Below is an embodiment of the invention which will be described withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically an arrangement for supplying ammonia as areduction agent to an SCR catalytic converter disposed in the exhaustduct of an internal combustion engine including a valve disposed in thesupply line and shown in an open position; and,

FIG. 2 shows the valve in a closed position.

DESCRIPTION OF A PREFERRED EMBODIMENT

A device for supplying ammonia (NH₃) to a reduction catalytic converterdisposed in the exhaust duct of a diesel engine of a motor vehicle isindicated in FIG. 1 overall by the reference numeral 1. The device 1comprises a container 2 for generating ammonia (NH₃) by thermolyticdecomposition of an NH₃ precursor, wherein a pressed body 3 of ammoniumcarbonate is used in the shown embodiment as NH₃ precursor. The ammoniumcarbonate body 3 is cylindrical and is formed to the shown shape bycompressing powdered ammonium carbonate. The pressed ammonium carbonatebody 3 has a round cross-sectional area. The container 2 includes aheating device which is designated by the reference numeral 4. Theheating device 4 is in the form of a radiation heater.

The radiation heater is formed in the shown exemplary embodiment bythree heating elements 5, 5′ 5″ which are each in the form of anArchimedes spiral and banked relative to one another. The individualheating elements 5, 5′, 5″ of the heating device 4 are controllableindependently of one another so that the heating energy generated and/oralso the course of a heating phase can be controlled. The heating device4 is arranged in a lower part of the container 2. Below the heatingelements 5, 5′ 5″ a heat barrier 6 is arranged. Above the radiationheater and at a small distance therefrom there is a heating plate 7which forms another part of the heating device 4. The heating plate 7consists in the shown embodiment of a transparent glass-ceramic materialthrough which the heat radiation generated by the heating device 4 canpass. The side of the heating plate 7 facing the pressed ammoniumcarbonate body 3 is surface structured with knobs. At its lower surfacefacing the container bottom, the heating plate 7 is provided with a tube8, which forms a connecting channel 9. The connecting channel 9 isconnected to a supply line which is generally designated by thereference numeral 10.

FIG. 1 shows the device 1 and, in particular, the container 2 with thepressed ammonium carbonate body 3 disposed therein before the initialoperation thereof but with the supply line 10 already open. At thispoint, the lower surface of the pressed ammonium carbonate body 3 isdisposed on top of the knob structure of the heating plate 7. With thefirst operation of the device 1, upon energizing the heating device 4, acertain amount of the ammonium carbonate body 3 is thermolyticallydecomposed. As a result, the knobs extend onto the ammonium carbonatebody 3 and fix the body 3 in place.

The heating device is so designed that at the side of the heating plate7 facing the ammonium carbonate body 3 the maximum temperaturesgenerated are lower than the decomposition temperature of the reactiongas or gas mixture formed during the thermolysis of the ammoniumcarbonate. This is achieved to a large extent by the glass-ceramicheating plate 7. It allows the heating device 4 to combine theadvantages of a radiation heater with regard to a rapid reactioncapability and the incidental spontaneous decomposition of ammoniumcarbonate with the advantages of heating devices with heat transfer bydirect contact.

The container 11 is provided with a rolling sleeve piston 11 whichdivides the interior of the container into a first container section anda second container section. By the rolling sleeve piston 11, the twocontainer sections are separated in a gas-tight manner. The firstcontainer section contains the heating device 4 and the pressed ammoniumcarbonate body 3 as well as the outlet formed by the connecting channel9. The second container section is connected to a compressed air systemD. By way of the compressed air system which also includes a compressorproviding the needed compressed air to the system a pressure isestablished in the second section as it is needed for establishing thedesired engagement pressure between the pressed ammonium carbonate body3 and the upper side of the heating plate 7.

In the container 2, the solid pressed ammonium carbonate body 3 isstored as NH₃ precursor. Upon energization of the heating device 4, theammonium carbonate disposed on the heating plate 7 is decomposed so thatthe NH₃ precursor is converted to the second phase—the gaseous phase.The gaseous reduction agent is supplied via the connecting channel 9 andthe supply line 10 to the exhaust duct as, respectively, the SCRcatalytic converter.

The supply line 10 includes a valve 12 which is shown in FIG. 1 in anopen position. The valve 12 comprises as movable element a Peltierelement 13 which can be moved by an activating member 14 to the openposition. The activating number 14 may be for example an electromagnet.The valve can be moved into the closed position as shown in FIG. 2 bythe Peltier element 13 using the energy stored in the compression spring15. The surface 16 of the Peltier element 13 forms the movable valvearea and consequently the control element of the valve 12. A membrane 17circumferentially seals the valving chamber 18. In the open position ofthe valve 12, the reaction gas generated in the container 2 by thethermolysis flows through the valving chamber 18 into the supply line 10via the valve 12 which is in communication with the exhaust duct inorder to supply the gaseous reduction agent to the SCR catalyticconverter. When the valve 12 is closed, as shown in FIG. 2, the Peltierelement is at the same time energized for cooling the heat exchangesurface 16 thereof. By the abutment of the surface 16 of the Peltierelement 13 at the valve opening for the connecting channel 9 and thechannel extending from the valve 12 to the exhaust duct with both valveopenings being sealed by seals 19 with respect to the surface 16, thechannels adjacent the valve 12 form cold traps so that reduction agentgas still present in the two channels is drawn to the cold surface 16closing the channels. As a result the channels 10 are kept open as thechances of a reformation of ammonium carbonate from the gaseous reactionagent in the supply line 10 is avoided to a large degree. Upon openingthe valve 12 or shortly before opening, the Peltier element is soenergized that it becomes a heating element whereby the ammoniumcarbonate deposited on the surface 16 during cooling thereof is rapidlydecomposed and the valve becomes operative for controlling the gaseousreaction agent flow.

In the embodiment described in the Figures, the two flow opening of thevalve 12 with the seats 19 are arranged side-by-side. Both openings aretherefore closed at the same time by the surface 16 of the Peltierelement 13 which serves as a valve control surface. The Peltier element13 is energized for cooling expediently only shortly after the valve 12has been closed.

Ammonium carbonate is the preferred NH₃ precursor material for operatingthe device as described. It is advantageous in connection with the useof ammonium carbonate that its thermolytic decomposition begins to alarge extent already at temperatures above 70° C. This relatively lowtemperature has the advantage that the ammonium carbonate deposited onthe valve surface 16 during cooling thereof can again be decomposed withrelatively little energy input for freeing the valve from the ammoniumcarbonate deposited and reformed on the surface 16 during cooling as theammonium carbonate is again rapidly decomposed.

1. A device for keeping a valve controlled flow passage supplying areduction agent to a catalytic converter in an exhaust system of adiesel internal combustion engine, said reduction agent being gaseousabove a certain temperature, but solid below said certain temperature,said valve (12) being a control valve for dosing the flow of the gaseousreduction agent to the catalytic converter, said control valve includinga valve seat (19) and a movable valve surface (16) which abuts the valveseat when the valve is closed, said valve surface (16) being formed by acontrollable element which, when energized is selectively cooled orheated so that, upon energization of the controllable element with thevalve closed and said valve surface being cooled, a cold trap structureis formed in the flow passage adjacent the valve surface (16) forsolidifying gaseous reduction agent present in the flow passage on thecold valve surface and, upon subsequent energization of the controllableelement, the solidified reduction agent is again converted to a gaseousphase, and a control unit for selectively controlling the energizationof the controllable element for cooling or heating the controllableelement.
 2. A device according to claim 1, wherein the movable valvesurface (16) is in the form of a heatable condensation element (13). 3.A device according to claim 1, wherein the valve surface (16) is formedby a heat exchange surface of at least one Peltier element (13).
 4. Adevice according to claim 1, wherein the valve (12) includes astationary valve seat provided with two adjacent connecting openings, ofwhich are in communication of a container (2) in which the reductionagent or a reduction agent precursor is stored and the other connectingopening is in communication with exhaust duct of the diesel engine.
 5. Adevice according to claim 4, wherein the closed portion of the valve(12) both connecting openings of the valve seat are covered by the valvesurface (16).
 6. A device according to claim 5, wherein the container(2) which is in communication with one of the connecting openings of thevalve (12) includes a precursor (3) compound which, when heateddecomposes to form the reduction agent or reduction agent mixtureincluding NH₃ and which, with a reduction of the temperature thereof isreformed to a solid state, said container (2) including a heatingstructure (4) for causing the thermolytic decomposition of the precursorcompound wherein NH₃ is formed for supply to the catalytic converter inthe exhaust system.
 7. A device according to claim 6, wherein the NH₃precursor stored in the container (2) is pressed ammonium carbonate body(3).